有关ADCB实验的文章

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1. Introduction
Block copolymers have been widely used to reinforce interfaces between immiscible polymers.'$ The reinforcement mechanism is believed to be 'stitching"; i.e., block copolymerchainsform interphase junctions through which stress can be transferred, which results in substantial reinforcement of the interfaces. Creton et al.3 examined this reinforcing effect of dPS/PVP block copolymers and concluded that a t least one entanglement between each block and its respective homopolymer was necessary to achieve effective interfacial reinforcement. The reinforcement effect of block copolymers was measured by the fracture toughness increases of the PS/ PVP interfaces. Recently, an asymmetric double cantilever beam specimen has been developed4r5and applied to measure the fracture toughness of interfaces between immiscible polymers and the interfaces between polymer and nonpolymer materials6$ (Figure 1). In these studies, it was found that changing the thickness ratio of the two beams in the ADCB specimen results insignificant changes in the measured fracture toughness of the same interfa~e.~.I-$ In some cases, the crack can be driven out of the interface by changing the thickness ratio, resulting in cohesive failure of PS or PVP instead of adhesive failure alongthe interface. Changing the thickness ratio changes the phase angle which controls the ratio of the shear/ normal traction directly ahead of the interface crack tip. To quantify the improvement in the fracture toughness of the interfaces as a function of polymerization index of both blocks, Nps and Npw, and the areal density of the block copolymer chains, 2 , our previous experirnental~8J0 were conducted with a fixed phase angle of about -6". which corresponds to a fixed PS/PVP thickness ratio of 2.3D.6, so the crack remains on the interface as it is driven toward the more craze resistant PVP. The objective of this work is to determine the dependence of interface , on the phase angle J. and how that fracture toughness G
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Macromolecules 1994,27, 4382-4390
Phase Angle Effects on Fracture Toughness of Polymer Interfaces Reinforced with Block Copolymers
Fei Xiao and Chung-Yuen Hui' Department of Theoretical and Applied Mechanics and the Materials Science Center, Cornell University, Zthaca, New York 14853
Junichiro Washiyama' a n d Edward J. Kramer Department of Materials Science and Engineering and the Materials Science Center, Cornell University, Ithaca. New York 14853 Received January 5, 1994. ABSTRACT The effect of the phase angle on fracture toughness and fracture mechanisms of planar interfaces between polystyrene (PS) and poly(2-vinylpyridine) (PVP) reinforced with deuterium-labeled polystyrene (dPS)/PVP block copolymers was investigated using an asymmetric double cantilever beam (ADCB) specimen. The fracturetoughness of an interfacialcrack, C,, was measured as a function of the phase angle J., the areal chain density of the block copolymer X, and the degree of polymerization N of the chains. The fracturemechanismsoftheinterface werestudied by forwardrecoilspectrometry(FRES), which permitted thelocationofthedPSblocktobedeterminedafterfracture. Thereisadramaticchangeinfracturetoughness as the phase angle changes sign, a change which is associated with the formation of crazes in the PS at, and away from, the interface. The angle B of craze propagation away from the interface depends on the phase angle For positive phase angles, B is close to 45O; for negative phase angles, 0 is close to 135'.
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Figure 1. ADCB specimen with PVP as the top beam.
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dependence is altered when E and the polymerization indices of the block copolymer are changed. As in previous experiments, we chose polystyrene (PS) and poly(2-vinylpyridine) (PVP) homopolymers as the immiscible polymer pair and dPS/PVP block copolymers to reinforce the interface between PS and PVP. There are several advantages of this system. Both PS and PVP deform plastically by crazing,' which can be investigated using transmission electron microscopy (TEM) techniques." The PS phase has a smaller crazing stress than PVP, so it will craze before PVP does. Furthermore, thermodynamic characteristics, such as the Flory interaction parameter, x , between PS and PVP are known from previous studies,'%" giving us an additional advantage in understanding this system. Accordingto the deformation-fracture mechanism map proposed by Xu et al.15 and Washiyama et al.? two transitions of fracture mechanism are possible as 2 increases. The first, for long blocks (Le., both Nps and Npw >> Ne, where Ne denotes the polymerization index between entanglements of its respective homopolymer1), is the transition from chain scission (with negligible crazing) tocrazing(fo1lowedby chainscission in thecraze). Thesecond is the transitionfromchainpull-out tocrazing (e.g., when Npvp < Ne for the PVP block).8